Bad focus is one of the most common causes of unusable photos, along with camera shake and incorrect exposure.

Understanding and correctly using autofocus tends to be somewhat more technical than avoiding camera shake, but it’s critical to good photo technique. Once you have a basic understanding of how to avoid common focus problems, your photos will look sharper.

Autofocus basics

Autofocus is now pervasive. Virtually every currently made camera includes some form of autofocus mechanism. There are two types of autofocus hardware, contrast detection and phase detection. Both have advantages and disadvantages, and both require lenses specifically designed for auto-focusing with that particular camera.

Many interchangeable-lens cameras also allow you to use older, strictly manual-focus lenses, either directly or with an adapter. Manually focusing almost any lens can be extremely accurate when done correctly. Although older, film-era, manual-focus lenses are sometimes quite good, overall, the image quality of film-era lenses is not always up to that of modern aspherical lenses designed specifically for smaller digital sensors.

Fixed-lens cameras build the complete autofocus mechanism directly into the integrated camera and lens combination. Interchangeable-lens cameras require that each lens include whatever autofocus hardware is needed to mesh with that camera.

Even within the same camera brand, it’s important to be sure your lenses and cameras are compatible. Not uncommonly, older autofocus lenses can be autofocused only by a small motor in the camera body.

Newer autofocus lenses often place an autofocus motor within each lens but they usually work with a camera’s built-in autofocus motor if the lens and camera are otherwise compatible. Before you buy, verify compatibility between lenses and camera bodies. When in doubt, buy a camera body that includes a built-in autofocus motor.

Entry-level dSLR cameras often reduce manufacturing costs by omitting any autofocus motor in the camera body. That omission forces you to use only newer lenses with embedded autofocus motors with that camera body. Although embedded autofocus motors often focus faster and more quietly, a real plus when making video, they’re less robust and potentially more likely to fail. They’re also more expensive because every lens must include its own complete mechanism, increasing your total cost if you buy several lenses.

Personally, I prefer a camera body that includes an autofocus motor that allows me to choose among a wider variety of good lenses. Because I mostly take still photos rather than video plus sound, I’m not bothered by the camera motor’s slight noise. Your own preferences may differ.

Phase detection autofocus is used in almost all dSLR cameras and requires a separate focus detector that receives light reflected from the optical viewfinder’s mirror. This is a very fast type of autofocus, particularly useful for low-light shots, for low-contrast subjects — like clouds that have no sharp boundaries — and for fast-moving subjects.

The disadvantage is that phase detection requires very precise alignment of the focus detector and mirror so that they are exactly the same distance as the digital sensor. When the focus detector’s alignment is not assembled perfectly, then focus is not perfect, either. When a dSLR lens focuses in front of the intended subject, it’s called front focus. When the lens focuses too far behind the subject, it’s called, naturally, back focus.

Depending on the lens in use, the same dSLR camera may focus perfectly on the intended subject, in front of the subject or to the rear. Sometimes it may exhibit all three focus modes in the same camera when using different lenses.

Contrast detection autofocus is the method used by compact consumer cameras and mirrorless compact system cameras. Contrast detection focus uses focus detectors embedded directly in the imaging sensor, moving the lens back and forth until image contrast is maximized at the selected focus point. As a result, this autofocus method tends to be fairly reliable with high-contrast images, less subject to faulty assembly alignment, but more prone to other problems.

Most better dSLR cameras now allow you to use a live view mode, composing with the rear LCD panel rather than through the viewfinder. When using live view modes, your dSLR camera will usually switch its autofocus mode to sensor-based contrast detection.

Some newer contrast detection autofocus systems also include a focus peaking indication where the area in sharpest focus is outlined in another color on the LCD panel’s image. Focus peaking is a very nice feature, but it’s usually available only on the most expensive mirrorless compact-system cameras.

However, as with phase detection autofocus, contrast detection has some limitations, the most serious of which was very slow performance. Recently, though, Olympus and Panasonic have used a new, very fast type of autofocus hardware in a few of their upper-end compact system cameras, like Olympus’ E-P3 and Panasonic’s GH2. The autofocus performance of these cameras is finally on par with dSLR cameras. A few levels lower, though, and contrast detection autofocus remains very slow, scarcely suitable for fast-moving subjects like sports.

Contrast detection autofocus often fails to focus at all when pointed at inherently low-contrast subjects, like clouds. With such subjects, you may need to switch to an electronically magnified manual focus using the rear LCD, or manually set the focus based on your estimate of distance.

Calibrating autofocus

Many upper-end dSLR cameras include the ability to individually adjust how each lens is focused, allowing you to compensate for front focus or back focus. Calibrating your dSLR camera so that every lens actually focuses on the indicated point takes some time and testing. However, the effort is worthwhile because you may see real improvement if your dSLR’s focus detector is a bit off for one or more of your lenses. Although time-consuming, it’s not a difficult process and there are some simple tools that help you check your camera’s focus for each lens.

The best commercially available tool is the LensAlign MkII Focus Calibration System ($80 at Amazon.com). Although not inexpensive, LensAlign is well-made and precise. It’s much less expensive than replacing camera bodies and lenses, then hoping for the best. Before ordering LensAlign, be sure your dSLR camera allows you to individually adjust the focus settings for several lenses. You can find a free, print-it-yourself focus adjustment scale at http://pentaxdslrs.blogspot.com/2012/01/front-or-back-focusing-problems-free.html, but the commercial LensAlign tool is superior.

Using autofocus

Every camera implements autofocus features differently. Most cameras, though, have at least five to nine different autofocus points, with a central focus sensor and several other sensor points arranged around the central point. Some professional cameras include several dozen focus sensor points.

How a camera uses its autofocus points in a particular situation determines where it will focus and what will, or will not, be in focus. When using default settings, a camera’s choice of focus point is largely an automatic process that’s preprogrammed at the factory, for better or worse.

You can, however, exercise a great deal of control over autofocus, and that’s often the difference between a sharp photo and one that’s out of focus. Many better dSLR cameras have an external dial or button that allows you to easily alter the default pattern of active autofocus points. Compact consumer and compact-system cameras generally require you to enter the setup menu to change focus settings.

Typical autofocus patterns include using only the central autofocus point, using a small pattern of points clustered closely around the center, or allowing the camera to make its choice from all possible focus sensor points. In some cases, as with my Olympus E-P3, you can use the touch screen or access a menu to force the use of a specific autofocus point.

Most better cameras allow you to depress the shutter button halfway, which causes the camera to choose a specific autofocus point and bring the subject into focus. Your viewfinder or LCD screen will then light up with a small red dot showing which autofocus point was chosen. If it’s the part of the photo that you want in sharpest focus, then continue making the exposure. Autofocus has been good to you in this instance.

If the red dot is not on whatever should be in sharpest focus, then either use the menu or touch screen to choose the correct focus point or switch to the center focus point. Put the center point on your subject, depressing the shutter release halfway to lock that focus. While holding the shutter button halfway down, reframe your photo to its correct appearance, and then fully depress the shutter button. Alternatively, you can hold the correct focus by pressing the autofocus lock button if your camera has one handy.

Generally, I prefer to use either center autofocus point, locking it until the exposure is made, or the small pattern of autofocus points clustered around the center.

Creativity in technology

Rather than dry technical details, let’s finish with a thought about the creative process. I recently came across a nice quote from my own fine art photography instructor at MIT, Minor White, who certainly knew both his technology and his creativity.

“The state of mind of the photographer while creating is blank. … For those who would equate ‘blank’ with a kind of emptiness, I must explain that this is a special kind of blank. It is a very active state of mind, really, a very receptive state of mind, ready at an instant to grasp an image, yet with no image preformed, pattern or preconceived idea of how anything ought to look is essential to this blank condition. Such a state of mind is not unlike a sheet of film itself — seemingly inert, yet so sensitive that a fraction of a second’s exposure conceives life in it.”

This sort of blank-slate open-mindedness is just as important to the viewer of a photograph as it is to the photographer, for there is just as much creativity, and need for creativity, in the viewer. So, the next time that you happen to see any sort of artwork, photograph or otherwise, try to open your mind without preconception as you view it. You might be pleasantly surprised.

Local attorney Joe Kashi received his bachelor’s and master’s degrees from MIT and his law degree from Georgetown University. He has published many articles about computer technology, law practice and digital photography in national media since 1990. Many of his technology and photography articles can be accessed through his website, http://www.kashilaw.com.